In the measurement art the aim of measuring deformation phenomena such as bending, stretching, upsetting and the like with a high degree of accuracy arises in many different ways and situations. One form of apparatus for measuring deformation of a body, as is to be found in DE 43 32 807 A1, provides a light exit face for radiating light and a light entrance face which can be connected to an evaluation unit, for receiving at least a part of the light emitted by the light exit face. The light exit face and the light entrance face are mounted on the body in such a way that the position thereof relative to each other alters upon deformation of the body. In that arrangement the body has a through bore, into which a transmitting optical fiber providing the light exit face is inserted from one side while inserted into the through bore from the other side is a receiving optical fiber affording the light entrance face, so that the light exit and entrance faces are disposed in mutually opposite relationship at a spacing from each other. When deformation of the body occurs the amount of light which passes from the light exit face into the light entrance face alters. In a further construction the body which can be fixed for example by way of feet and clamping straps to the tail portion of a pair of welding tongs is formed by a plate which carries two blocks at a spacing from each other, with a weakening line between the blocks. One block is provided with two through bores which extend at an angle relative to each other and whose axes intersect at a mirrored surface which is provided on the other bore. A transmitting optical fiber is fitted into one bore and the receiving optical fiber is fitted into the other bore. Upon deformation of the carrier plate the amount of light which passes from the transmitting optical fiber by way of the mirrored surface onto the receiving optical fiber alters.
A particularity of the above-indicated apparatus or opto-electronic sensor is that the measurement signal not only depends on the deformation of the main body, but is in addition proportional to the amount of radiated light. As a result, a calibration operation is required before each measurement procedure. In addition, it is not possible to infer from the measurement signal the direction of deformation of the body, and therefore such direction has to be determined by additional measuring operations, with their attendant costs.